The present invention relates to a ballast circuit for a gas discharge lamp. More particularly, the present invention relates to a ballast circuit operated directly from an alternating current (A.C.) voltage source and having a capacitor serially connected to a serially arranged incandescent filament and a gas discharge tube.
Recent improvements to the incandescent art have provided an improved lighting unit having a highly efficient gas discharge tube as the main light source and an incandescent filament as a supplementary light source. Such an improved incandescent lamp is generally described in U.S. Pat. No. 4,350,930, of Piel et al, issued Sept. 21, 1982.
The gas discharge tube has various modes of operation such as, (1) an initial high voltage breakdown mode, (2) a glow-to-arc transition mode, and (3) a steady state run mode. One of the circuit performance parameters is that the voltage applied across the gas discharge tube be such that the current flowing within the gas discharge tube is maintained above a critical value such as 60 milliamps. If the current flowing in the gas discharge tube drops below this critical value the arc condition of the gas discharge tube may extinguish, which, in turn, may cause the gas discharge tube to revert from its steady state run mode to its glow-to-arc transition mode or even to the initial breakdown mode. The reestablishment of the desired arc condition of the gas discharge tube may require a restrike voltage having a voltage value typically about 2.5 times or more than that of the operating voltage of the gas discharge tube.
The restrike voltage necessary for a gas discharge tube of 2.5 times its operational voltage presents a difficulty for a ballast circuit for a discharge tube operating directly from a 120 volt, 60 Hz A.C. source. For example, if the gas discharge tube has an operating voltage of 80 volts A.C. a restrike voltage of 80.times.2.5=200 volts or more is typically necessary and which voltage value is not ordinarily available from the peak-voltages of a typical 120 volt, 60 Hz A.C. source. It is considered desirable to provide means for reducing the operating voltage of a gas discharge tube, which, in turn, reduces the value of the necessary restrike voltage, which, in turn, more readily allows development of the restrike voltage from the peak voltage value of a typical 120 volt, 60 Hz A.C. source, which, in turn, more readily allows the ballast circuit to operate the gas discharge tube directly from a 120 volt, 60 Hz A.C. source.
A further difficulty involved with a ballast circuit is its ability to adapt to changes in the voltage and frequency parameters of the A.C. source. The voltage and frequency parameters are determined by the available power source. For example, the circuit parameters of a ballast circuit are typically selected for the applied A.C. source so that a ballast circuit operating with an applied A.C. source of 120 volts, 60 Hz does not perform in a successful manner when the applied A.C. source is changed from a 120 volt, 60 Hz A.C. source, typically available for U.S. utilization, to a 220 volt, 50 Hz A.C. source typically available for European utilization and elsewhere in the world. It is considered desirable to provide a ballast circuit for an gas discharge tube operable directly from either a 120 volt, 60 Hz A.C. power source or with suitable component selection for a 220 volt, 50 Hz power source.
Accordingly, objects of the present invention are to provide (1) a ballast circuit directly operable from an A.C. source and (2) to provide such a ballast circuit which directly operates with either 120 volts, 60 Hz A.C. power source or a 220 volt, 50 Hz A.C. power source.